Troubleshooting, or What the Hell Is That?

[Ari]

Ahoy! I thought I'd add a specific place where we can troubleshoot.
As a first example, I've been getting ridges on some plates (not all of them).



I was first told it's my collodion losing alcohol (I use the same bottle to pour excess back into).
So I added 40mL of alcohol to about 200mL of collodion, but the ridges still happen, though again, not every time. About 3 out of 4 plates.

Could it be my developer pour?
I use a tray. I place the plate at one end, I pour developer at the other end, then tilt the tray to wash developer over the plate.
Development is a constant 15s.

I clean my holder after each plate, wiping it down with a paper towel.

Any help appreciated, thanks.